Prior patents, such as U.S. Pat. No. 6,252,971 for “Digital watermarking using phase-shift stoclustic screens,” by S. Wang, hereby incorporated by reference in its entirety, describe a method to embed correlation-based phase-shift digital watermarks into halftone screens. By overlaying a transparency on the prints generated by the special halftone screen, for example as a public key, invisible watermarks can be retrieved. For color images, the watermark was embedded into all or a selected color channel in a dot-on-dot arrangement.
The present disclosure characterizes an improved system and method whereby different correlation-marks are embedded into different color channels, or layers. Therefore, watermarks are embedded and retrieved in multiple layers. This modification enables the use of the digital watermarking process to create a greater number of encoded images within a single image. In one embodiment, the different colors are printed at rotated angles, which further provide moiré-free halftone outputs and avoids the color instability arising from a dot-on-dot rendering approach. The overlay-transparencies, or the public keys, to retrieve the correlation-marks for different colors can be different in frequencies and/or angles, so different users with different keys can read different messages from the same printed output image.
One example is a case where three color channels (cyan, magenta, black) are halftoned with the same frequency amplitude but different angles, so the same public key, manifested for example in a transparency printed with a selected halftone screen pattern, can be used to view or retrieve different watermarks embedded in different colors with varying rotations/orientations of the transparency. The present disclosure provides potential applications of digital watermarking to digital imaging methods and products.
Disclosed in embodiments herein is a method for digital watermarking of a multi-color image, comprising: receiving the image to be watermarked; determining the watermarks to be embedded in the image; embedding a plurality of invisible digital watermarks into the image, wherein a first watermark is embedded into a first color layer and a second watermark is embedded into a second color layer; and outputting the watermarked image containing the embedded invisible digital watermarks, wherein at least two of the color layers in the image include invisible digital watermarks therein.
Further disclosed in embodiments herein is a method for digital watermarking of a multi-color image, comprising: receiving the image to be watermarked; determining a first watermark, a second watermark and optionally a third watermark to be embedded in the image; embedding the first, second and third watermarks into the image, wherein the first watermark is embedded into a first color layer, the second watermark is embedded into a second color layer and the optional third watermark is embedded into a third color layer; and outputting the watermarked image containing the embedded invisible digital watermarks, wherein the image include invisible digital watermarks in at least three of the color layers therein.
Also disclosed in embodiments herein is a system for digital watermarking of a multi-color image, comprising: an input image source; image memory for storing the input image to be watermarked; watermark memory for storing the watermarks to be embedded in the image; and an image processor for embedding a plurality of invisible digital watermarks into the image, wherein a first watermark is embedded into a first color layer and a second watermark is embedded into a second color layer.
The various embodiments described herein are not intended to limit the invention to those embodiments described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope defined by the appended claims.